Do mass shootings cause more mass shootings? Research is divided.

 Two  mass shootings in California last week that killed dozens of people in public spaces, as well as a third attack days earlier that killed six family members in their home, have again raised questions about whether such violent incidents inspire new ones.

Data on mass public shootings going back to 1966 suggests such attacks are becoming more frequent, said James Densley, co-founder of the Violence Project, a nonprofit research center.

But it’s not yet clear if shootings that come close together in time are intertwined.

"You find even in random simulations, there are times when there’s a cluster and times when there are not," said James Alan Fox, a criminology professor at Northeastern University who serves on USA TODAY’s Board of Contributors.

Are mass shootings happening more often?

The frequency of fatal mass shootings has increased from 23 a year in 2010 to 27 a year through the end of 2022, based on five-year averages of data gathered by USA TODAY, The Associated Press and Northeastern University.

That includes incidents in which at least four people were fatally shot, excluding the shooter, and a variety of motives – from hate crimes to family violence and gang-related attacks.

The number of incidents categorized as caused by hate, terrorism or indiscriminate violence, which tend to draw significant public attention, has increased from two a year to five. Similarly, attacks in public places like schools, hospitals and malls have increased from seven a year to 10 a year.